The invention concerns an impermeable cooled closing plug for processing tubes, in particular in semiconductor manufacture.
By processing tubes, tubes are to be understood in which for example an oxidation process takes place, a diffusion process is carried out, or a depositing process takes place. Also, tempering processes are undertaken in such tubes. These processing tubes are as a rule made of quartz and where required are closed with a cover.
The quartz tube of a known high pressure oxidation furnace is provided with a tube closing plug of steel. The tube closing plug serves the purpose of closing the quartz tube as tightly as possible, in order, among other things, to prevent the penetration of impurities from the surrounding environment into the interior of the tube, which environment is under higher pressure. For example, two Viton (a trademark of E. I. DuPont de Nemours Corp. for a type of synthetic rubber) O-rings serve as a seal. With higher temperatures, however, sealing rings of that sort have only very limited service life. It is thus necessary to cool them. If cooling is accomplished with water, and the tube closing plug consists of steel--a good heat conductor--it cannot be prevented that the cooling remains limited only to the direct vicinity of the sealing rings. This has the result that a large part of the tube closing plug surface is colder than a condensation point of the steam. In the case of damp oxidation, the steam which is present condenses on these cold points. This leads to the formation of droplets which run back into the quartz tube. They can penetrate into the operation zone where, for example, they can significantly interfere with the oxidation. For evaporation, they require heat which is drawn from the surrounding environment. This causes the temperature profile and thus also the oxide growth to become non-uniform.
The inner side of such a tube closing plug made of steel is strongly attacked by processing gases. Very quickly a thick iron oxide layer forms which must be regularly removed.
It can also not be avoided that water drops which form on the tube closing plug take up a part of the iron oxide, transport it into the tube, and there lead to contamination.